The present invention relates to extracorporeal systems for oxygenating and pumping blood during cardiac surgery. More specifically, the present invention relates to an integrated oxygenator and pump system wherein the oxygenator includes an active element that enhances gas diffusion, the system being sufficiently compact for use in a sterile field.
Each year hundreds of thousands of people are afflicted with vascular diseases, such as arteriosclerosis, that result in cardiac ischemia. For more than thirty years, such disease, especially of the coronary arteries, has been treated using open surgical procedures, such as coronary artery bypass grafting. During such bypass grafting procedures, a sternotomy is performed to gain access to the pericardial sac, the patient is put on cardiopulmonary bypass, and the heart is stopped using a cardioplegia solution.
More recently, techniques are being developed, for example, by Heartport, Inc., Redwood City, Calif., that permit cardiac bypass grafting using an endoscopic approach, in which small access openings are created between the ribs and the bypass graft or heart valve repair procedure is performed guided by an image displayed on a video monitor. In the xe2x80x9ckeyholexe2x80x9d techniques developed by Heartport, the patient""s heart is stopped and the patient is placed on cardiopulmonary bypass. Still other techniques being developed, for example, by Cardiac Thoracic Systems, Inc., of Menlo Park, Calif., enable such bypass graft procedures to be performed on a beating heart.
As a consequence of this trend towards minimally invasive cardiac surgical techniques, the need to maintain adequate space within the sterile field surrounding the small access sites has become critical. Whereas in open surgical techniques the sternotomy exposed a relatively large surgical site that the surgeon viewed directly, minimally invasive techniques require the placement of endoscopes, video monitors, and various positioning systems for the instruments that crowd the sterile field and can limit the surgeon""s ability to maneuver. In recognition of the increasingly crowded environment in which a surgeon employing minimally invasive techniques must work, a need to miniaturize the equipment employed in xe2x80x9ckeyholexe2x80x9d cardiac surgical procedures has been recognized.
While improvements have been achieved with respect to many instruments employed in the surgical field, space-saving improvements to previously known cardiopulmonary systems have not kept pace. Such systems generally employ a series of discrete components, including a blood filter, blood reservoir, an oxygenator, a heat exchanger, a blood pump, and one or more control systems for controlling the various components. These components are typically coupled to one another in fluid communication using surgical grade tubing, and generally all of the components are maintained outside the sterile field. Such cardiopulmonary systems are generally coupled to the patient using central cannulation sites, e.g., via the vena cava or right atrium and the aorta, using lengthy tubes that extend through the sterile field and may further restrict the surgeon""s ability to maneuver.
A further drawback of previously known cardiopulmonary systems is that in those systems the tubes connecting the patient to the device may constitute a relatively large volume. Consequently, such systems must be primed either with transfused blood products or saline, thus potentially compromising the patient""s immune system, diluting the patient""s blood, or both. In addition, such previously known systems comprise large non-native surface areas and increase the risk of further jeopardizing the patient""s immune system.
In recognition of some of these disadvantages of previously known cardiopulmonary systems, attempts have been made to miniaturize and integrate some of the components of cardiopulmonary systems. U.S. Pat. No. 5,270,005 to Raible describes an extracorporeal blood oxygenation system having an integrated blood reservoir, oxygenator, heat exchanger, pump and pump motor that is controlled by cable connected to a control console. In the embodiments described in that patent, venous blood passes into a reservoir, and then through a filter, a pump, and a static array of hollow fibers for oxygen/carbon dioxide exchange before being returned to the patient. U.S. Pat. No. 5,266,265 to Raible describes a similar system.
While the foregoing patents provide integrated blood oxygenation systems having relatively compact size and reduced priming volume, those systems rely upon relatively short flow paths through the oxygenator to provide adequate oxygenation of the blood. As is well recognized in the prior art, however, oxygenators having short flow paths may provide inadequate gas exchange, due to the development of laminar flow zones adjacent to the exterior of the gas exchange elements.
Whereas laminar flow zones develop in most previously known oxygenators, the large size of the gas permeable fiber bundles used in those devices generally enable adequate mass transfer for oxygen and carbon dioxide. The compact size and static nature of the oxygenators describe in the foregoing Raible patents, however, may lead to the development of laminar flow zones and stagnation zones that impede adequate oxygen and carbon dioxide exchange. One solution to lengthen the flow path for an integrated system is described in U.S. Pat. No. 5,411,706 to Hubbard et al. The system described in that patent recirculates blood through the fiber bundle at a higher flow rate than the rate at which blood is delivered to the patient.
Apart from the recirculation technique employed in the Hubbard et al. patent, other methods are known for interrupting the development of laminar flow zones. U.S. Pat. No. 3,674,440 to Kitrilakis and U.S. Pat. No. 3,841,837 to Kitrilakis et al., which are incorporated herein by reference, describe oxygenators in which an active element stirs the blood within the oxygenator, thereby disrupting the development of laminar flow zones and enhancing mass transfer. Despite favorable test data indicating that such xe2x80x9cactivexe2x80x9d systems do not enhance shearing damage to the blood cells, as reported, for example, in an article entitled xe2x80x9cA Rotating Disk Oxygenator,xe2x80x9d Artificial Lungs For Acute Respiratory Failure, Academic Press, pp. 211-222 (W. Zapol ed. 1976), that technology has nevertheless been largely abandoned.
In view of the foregoing, it would be desirable to provide a compact extracorporeal blood oxygenation system that provides compact size, low priming volume, low surface area, and the ability to adequately oxygenate blood using an active element that disrupts the formation of laminar flow zones and stagnation zones with the fiber bundles of the oxygenator.
In also would be desirable to provide an integrated extracorporeal blood oxygenator and pumping system having a low priming volume and low internal surface area, thereby reducing blood contact with non-native surfaces, potential damage to blood components, and the risk of infection.
In addition, occasions arise during bypass surgery where it may be desirable to alternate between providing oxygenated blood and blood pumping. For example, in the beating-heart minimally invasive surgical methods developed by Cardio Thoracic Systems, the patient may not be placed immediately on cardiopulmonary bypass. Nevertheless, it may be desirable to use a pump to reduce the load on the heart. At a later stage of the surgery, it may be desirable to rapidly switch from a pump-assisted, beating heart method of surgery to a method involving stopping the patient""s heart and placing the patient on full cardiopulmonary bypass.
It would therefore be desirable to provide an integrated extracorporeal blood oxygenator and pumping system wherein the surgeon may select pump operation either with or without inclusion of the blood oxygenator in the fluid circuit.
In addition, it is common practice to maintain a cardiopulmonary unit on standby in an operating room during use of beating heart cardiac bypass grafting procedures, and more complicated angioplasty procedures, to enable rapid conversion to open surgical techniques should complications develop during a procedure.
It further would be desirable to provide an integrated blood oxygenation and pump system having the capability to provide pump-only capacity, but which enables the patient to be placed on full cardiopulmonary support almost immediately.
It s till further would be desirable to provide an integrated extracorporeal blood oxygenator and pumping system having a low priming volume, making the system suitable for emergency back-up operation.
In view of the foregoing, it is an object of the present invention to provide a compact, integrated extracorporeal blood oxygenation and pump system that provides small size, low priming volume and the ability to adequately oxygenate blood using an active element that disrupts the formation of laminar flow zones and stagnation zones with the fiber bundles of the oxygenator.
It is another object of the present invention to provide an integrated extracorporeal blood oxygenator and pumping system having a low priming volume and low internal surface area, thereby reducing blood contact with non-native surfaces, potential damage to blood components, and the risk of infection.
It is yet another object of this invention to provide an integrated extracorporeal blood oxygenator and pump system wherein the surgeon may select pump operation either with or without inclusion of the blood oxygenator in the fluid circuit.
It is a further object of the present invention to provide an integrated blood oxygenation and pump system having the capability to provide pump-only capacity, but permits the surgeon to rapidly place a patient on full cardiopulmonary support should complications arise using a beating-heart cardiac bypass technique.
It is a still further object of the invention to provide an integrated extracorporeal blood oxygenator and pumping system having a low priming volume, making the system suitable for emergency back-up operation.
These and other objects of the invention are accomplished by providing an integrated blood oxygenation and pump system, suitable for use within a sterile field, having a low priming volume. In accordance with the principles of the present invention, the blood oxygenator includes an active element, separate from the pump, that disrupts the development of laminar flow zones and stagnation zones in the fiber bundle employed for gas exchange.
In a preferred embodiment, the integrated blood oxygenation and pump system includes a first compartment housing a pump coupled in fluid communication to a second compartment housing an active blood oxygenator. The active blood oxygenator includes an active element, separate from the pump, that enhances mixing within the blood oxygenator to reduce the development of laminar flow zones and provide adequate oxygenation of the blood, without the need for recirculation. The pump may have any one of a number of suitable configurations, and may be axial, centrifugal, roller-type or bladder-type.
In addition, the integrated system may include a valve that permits the pump to be used independently of the oxygenator, or be rapidly switched to provide both pumping action and blood oxygenation. The active oxygenator of the present invention may also be used independently of the blood pump for situations where a low flow rate is acceptable, such as in pediatric cardiac surgery.
In one embodiment, the active element comprises a rotating disk comprising hollow fibers that carry oxygen to, and carbon dioxide from, blood contacting the fibers. In alternative embodiments, the fibers are fixed within the blood oxygenator compartment, and the active element comprises an impeller that sweeps over the surfaces of the fibers to mix the blood or a movable diaphragm that agitates the blood within the oxygenator. In a yet further embodiment, the hollow fibers of the oxygenator are mounted on a disk that is agitated with an oscillatory or vibratory motion.